(1) Field of the Invention
The present invention relates to a biaxially-stretch-blow-molded container and to a method of producing the same. More specifically, the invention relates to a heat-resistant polyester bottle of the one-piece type having excellent strength in the bottom portion, heat resistance, symmetrical panel-sinking stability in the vacuum pressure and self-standing stability and to a method of producing the same.
(2) Description of the Prior Art
Biaxially-stretch-blow-molded containers of a thermoplastic polyester such as polyethylene terephthalate (PET) have excellent transparency and luster on the surface, as well as excellent shock resistance, rigidity and gas barrier property that are required for the bottles, and have been used as containers, i.e., bottles for containing a variety kinds of liquids.
A hot filling method has been employed for bottling or packaging the contents maintaining enhanced preservability. Therefore, the containers must have heat resistance to withstand high-temperature liquids that are filled therein or the heat treatment that is carried out to sterilize the contents.
Heat-resistant containers are usually produced by a single-stage blow-molding method in which the blow-molded products and molded products are heated and crystallized, i.e., heat-set, using a single metal mold. With this single-stage blow-molding method, it is difficult to heat-set the recessed bottom portions of ordinary molded articles. To guarantee heat resistance, the recessed bottom portion must have an increased thickness and a complex shape. Accordingly, the weight of the bottom portion is subject to increase.
There has also been known a two-stage blow-molding method comprising a step for primarily blow-molding a preform article using a metal mold, and a step for finally molding the article by subjecting the article to the secondary blow-molding using a metal mold after the article is heat-shrunk via a so-called heat-set step through which the article is heated, shrunk and crystallized in an oven or the like.
In the two-stage blow-molding method, the article is drawn through the step of primary blow-molding to a sufficient degree at a drawing ratio of 2 to 6 times, and the thus drawn secondarily article is sufficiently shrunk by about 60 to 90% through the heat-set step, in order to improve rigidity of the bottle and to impart heat resistance based upon the intermediate heating. Therefore, the two-stage blow-molding method is suited for producing heat resistant containers.
Employment of the two-stage blow-molding method has been disclosed in, for example, Japanese Laid-Open Patent Publication No. 205124/1991 according to which a secondary article having a semi-spherical bottom portion is formed by the primary blow molding, the article is then heated in an oven, and the article is subjected to the secondary blow-molding to obtain a final article having a bottom of which the central portion is recessed inwardly of the container.
According to Japanese Laid-Open Patent Publication No. 189224/1988, furthermore, a secondary article is formed having a semi-spherical bottom formed by the primary blow molding, the bottom is then inverted to form a tertiary article having the bottom of which the central portion is recessed inwardly of the container, the tertiary article as a whole is heated relatively uniformly and is shrunk and, then, the article is subjected to the secondary blow molding to obtain a finally molded article.
According to the conventional two-stage blow-molding method, however, if some part is drawn at a large ratio in the final secondary blow molding step, then, the thickness of that part is-locally decreased to lose the strength or the degree of crystallization is decreased making it difficult to obtain heat resistance.
That is according to the former prior art, the drawing ratio increases at the bottom corner portion at the time when the secondary article having a semi-spheric al bottom portion is subjected to the secondary blowing to obtain a final product having the bottom portion that is inwardly recessed. Therefore, the thickness of the corner portion decreases and the degree of crystallization decreases resulting in a decrease in the strength and heat resistance at that portion.
According to the latter prior art, a semi-spherical bottom shape is obtained by the primary blow-molding and is inverted to obtain a secondary article having the bottom which is inwardly recessed. At this moment, the whole secondary article is heated relatively uniformly so that the height thereof is shrunk to be smaller than the size of the metal mold for secondary blow molding. In this case, however, the diameter of the barrel is extremely shrunk and when the heat-molded article having a small barrel diameter is subjected to the secondary blowing, the drawing ratio increases at the bottom corner portion to lose heat resistance and strength.
Moreover, when the heat-resistant container having a thick bottom portion of a recessed shape is obtained by the two-stage blow-molding method, the bottom of the secondary article formed by the primary blow molding has a thick recessed shape like that of the final product. When the bottom portion is heated like the barrel portion, the temperature rises slowly in the thick portion having a large heat capacity and portions other than thick portions are particularly heated. When the thus heated secondary article is subjected to the secondary blow molding, only those thin portions heated at a high temperature are drawn. In particular, the thickness of the bottom corner portion is extremely reduced, which is not desirable. Even when it is attempted to selectively heat thick portions of the bottom, the thick portions that are drawn at a small ratio in the primary blow molding are whitened. Usually, therefore, a complex heating system has been employed according to which the whole bottom portion of the secondary article is maintained at a moldable temperature without substantially causing it to shrink, and the barrel portion is chiefly heated, shrunk and crystallized.
When the thickness of the bottom portion is increased as described above, there arise problems in regard to that a complex system is required for heat-treating the secondary article and that the weight increases accompanying an increase in the thickness of the bottom portion. It therefore becomes necessary to solve such problems.
In order to prevent deformation of the bottle caused by the vacuum pressure after the heat-resistant bottle is filled with hot content, furthermore, a panel-rib structure has been widely employed in the barrel portion of the containers. In the container forming such a panel-rib structure in the barrel portion, the panel portion undergoes a paneling deformation inwardly by vacuum pressure. When the bottle is filled with hot content, however, the panel swells outwardly. Once the panel swells outwardly at the time of filling the content, the panel then loses its function to undergo paneling deformation inwardly. Therefore, the panel at the barrel portion is sometimes deformed asymmetrically.
According to the above-mentioned prior art, the bottle is greatly oriented on the inner side. Moreover, since use is made of a metal mold of a high temperature, the degree of heat set (degree of orientation and crystallization) increases on the outside of the bottle. When the bottle is filled with hot content, therefore, the panel portion swells out under certain circumstances.